warm nuclei - tumelements of nuclear physics • shell model (m.goeppertmayer, j.h.d. jensen, 1949)...
TRANSCRIPT
WARM NUCLEI
Helmut HofmannPhysik Department
TUM
Colloquium given at the University of Graz, Austria, 28.03.06
Table of contents• Elements of nuclear physics
– shell model vs compound nucleus
• Concept of nuclear temperature: false or true?– consequences of thermal isolation
• Transport theory – application in fission and heavy ion collisions– origin of irreversibility, nature of dissipation– fluctuating forces
• Summary, open problems
Elements of nuclear physics• shell model (M.GoeppertMayer, J.H.D. Jensen, 1949)
independent particle model: nucleons move in mean field, s.p. states group in shells, largely determined by appropriate spinorbit coupling
ground state 1particle 1hole 2p2h
Fermi energy: radius
• deformed shell model (A. Bohr und B. Mottelson, 1952)
nuclei are deformed: rotational modes vibrational modes
described by introducing shape variables Q(t)
as collective degrees of freedom
needed: appropriate equations of motion
treat motion of nucleons in timedependent, deformed mean field
assumption: nucleonic motion mast faster than collective
allows for easy generalization to motion of large scale like fission
deformed mean fields (parameterized by shape variables Q(t))
WoodsSaxonpotential
Cassini ovaloidsV. Paskevich
given density distribution folded with Yukawa interactionbecause of short range:
Berkeley Los Alamos group
potential deformation
follows that of densityselfconsistency condition
• compound nucleus (Niels Bohr, 1936)
basic assumption of shell model: mean free path much larger than nuclear dimension
cannot explain sharp resonancesfor slow neutrons
for motion across potential well widths of resonances would be of order MEV
Niels Bohr: all nucleons react with each other strongly
• compound nucleus (Niels Bohr, 1936)
assume: mean free path much smaller than nuclear dimension
additional nucleon stays inside nucleus for long time small ... narrow width
relaxation evaporation
hypothesis: final state independent of initial channel: intermediate state: microcanonical equilibrium
these features can be proven rigorously in nuclear reaction theory with only a few assumptions
V.Weisskopf, H. Feshbach
• liquid drop modelN. Bohr and F. Kalckar 1937:
semiclasical version of compound model:nucleus behaves like a liquid drop
(mean free path smaller than nuclear dimension)studied modes of a vibrating liquid drop
in accord with BetheWeizsäcker formula for static energy:volume term + surface term + curvature term + Coulomb
term
expansion in terms of 1/R or A^(1/3) macroscopic limit: no shell effects
commonly referred to as liquid drop energy
• shell correction for static energy: V.M. Strutinsky (1967)
= liquid drop energy+ shell correction
shell correction determined by contributions from first shell below and above Fermi energy
shells exist also for deformed shapes
shell correction determines ground state deformation and nuclear masses (with precision better than 0.5 per mille)
isomeric states at larger Q verified experimentally
principle points of view seen in various kinds of theoretical approaches (HartreeFock,periodic orbit theory)
shell correction method for static energy... combines two controversial approaches:
liquid drop model (picture of compound nucleus)and shell model (independent particles)possible justification: finite widths of quasiparticles away from Fermi energy
comparision of measured and theoretical values for finite nuclei
theoretical calculations for nuclear matter at T=0 and finite temperaturesC. Mahaux and R. Sartor
expect:consequences for shell effects
• temperature dependence of shell effects
disappear above T = 1.5 .. 2 MeV ... macroscopic limit is reached e.g.: Periodic Orbit Theory (POT)
example: free energy
left: numerical evaluation with realistic mean field (WS)right: simple formula
similar calculations for internal energy, entropy....
theoretically proven by various methods and authors, without considering finite widths of s.p. energies
definition of „warm“
• T < 3 5 MeV << 38 MeV (Fermi energy)• concept of mean field applicable• shell effects important• and hence, quantum effects for nucleonic motion• expect quantum effects for collective motion • intriguing problem of quantum transport
– isolated, small and selfbound system– interplay between collective and nucleonic d.o.f.– role of residual interactions
• discarded in HF, density functionals etc.
Table of contents
• Elements of nuclear physics– shell model vs compound nucleus
• Concept of nuclear temperature: false or true?– consequences of thermal isolation
• Transport theory – application in fission and heavy ion collisions– origin of irreversibility, nature of dissipation
• Summary, open problems
concept of temperature – applicable or not?
• introduced by H.A. Bethe in 1937 ..... (V.Weisskopf)
• used by H.A. Kramers in his theory of fission (1940)• later disliked and distrusted
– in studies of nuclear structure (shell model)– reaction theories
• presently in wide use, for different reasons– treated as mere, formal „parameter“ for
• level densities • studies of shell effects and their disappearance
– indispensable for applications of functional integrals
• in application of transport theories temperature has to have a real physical meaning (used since the 70‘ties)
possible definition of temperature
recall treatment of neutron induced reaction in statistical model:
relaxation evaporation
microcanonical equilibriumdetermined by
level densities entropiesin intermediate step:
possible definition of temperature
recall treatment of neutron induced reaction in statistical model:
relaxation evaporation
microcanonical equilibriumdetermined by
level densities entropiesin intermediate step:
try common definition of temperature ....canonical distribution
possible definition of temperature
microcanonical equilibrium evaporation
C B + b
study decay
possible definition of temperature
microcanonical equilibrium
decay rate
evaporation
C B + b
study decay
probability that subsystem b has energyin microcanonical ensemble
probability in canonical ensemble
(L. Szilard 1925)
concept of temperature – really applicable ???
PROBLEM: nucleus is not only thermally isolated
but also small
with specific heat C
temperature has inherent fluctuations
Physics Today:
H. Feshbach [(1987) 9] C. Kittel [(1988) 93] B.B. Mandelbrot [(1989) 71 ]
J. Lindhard, Niels Bohr Centennial (1986)
in 60‘ties: fundamental work by B.B. Mandelbrot (and later by L. Tisza)
estimate by Fermi gas model (macrosocopic limit)
entropy as function of excitation energy level density parameter
estimate by Fermi gas model (macrosocopic limit)
entropy as function of excitation energy level density parameter
order of magnitudes:
within this margin temperature concept justified
„measured“ specific heats
K. Kaneko and M. HasegawaPhys. Rev. C 72, 024307 (2005)
obtain level density vs energy from empirical information:
• counting (v. Egidy et al)• transfer reactions• neutron resonances• not possible for larger E (resonances overlap!!!)
Oslo group (E. Melby et al. PRL 83 (1999) 3150):
try to deduce specific heat as function of Tstructure seen in region wherepairing disappears (phase transition)
example: Tungsten isotopes
use
same order as width !!
negative specific heats (?)
at larger thermal excitations there might be a liquid gas phase transitionabove the „flash point“ free energy has no minimum anymoremuch before that heated nucleus expands, below a certain limit of density(about 1/3 to 1/6 of central density of stable nuclei) an assemblence of droplets is more stable than one big nucleus(„nuclear fog“, multifragmentation: nucleus splits into small fragments)similarity to galaxy formation (W. Thirring, H. Narnhofer ... Vienna)
specific heat may become negative (D. Groß (Berlin), P. Chomaz (Caen)...Coulomb force (long range) important (like in stars)similar effect seen in metal clusters (W. Thirring et al. PRL 91 (2003) 130601
microscopic theories for nuclear thermostatics
• Periodic orbit theory: (independent Fermions in box)
s.p. level density smooth part can be treated by Sommerfeld expansion
fluctuating part Fourier series BohrMottelson Vol.II (n=1)
Gutzwiller trace formula: classical action for orbits
calculation of all thermostatic quantities by cute expansion:P. Leboeuf et al. PRL 47 (2005) 102502
microscopic theories with residual interactions
• Shell Model Monte Carlo S.E. Koonin, D.J. Dean and K. Langanke, Physics Reports 278 (1997) 1 Y. Alhassid , G.F. Bertsch and L. Fang, PRC 68 (2003) 044322
• with functional integrals( Y. Alhassid, G. Bertsch, P.F. Bortignon, R. Broglia, P. Ring, R. Rossignoli)– residual interaction: series of separable forces– Hubbard Stratonovich to introduce mean field– „static path“: common free energy in mean field approximation– „perturbed static path“: fluctuations around it (local RPA)– in barrier regions: possible only above minimal temperature
(same problem as in „dissipative tunneling“)– partly overcome in thesis by C. Rummel (TUM 2004)
• expansion to fourth order• generalization of FeynmanKleinert variational procedure• later: decay rate for metastable systems with dissipation
Table of contents• Elements of nuclear physics
– shell model vs compound nucleus
• Concept of nuclear temperature: false or true?– consequences of thermal isolation
• Transport theory – application in fission and heavy ion collisions– origin of irreversibility, nature of dissipation– fluctuating forces
• Summary, open problems
(for BW:microcanonicalequilibrium evtlreduced to canonical)
diffusion coefficient in Einstein relation
temperature appears explicitely as aphysically real quantity
heavy ion reaction of fusionfission
approach phase:nuclei in ground state
overlap after contact: shape changesnucleons get heated
formation of compound nucleus:heated to maximal T
cooling by evaporationof neutrons emission of gamma's
fission
heavy ion reaction of fusionfission
approach phase:nuclei in ground state
overlap after contact: shape changesnucleons get heated
formation of compound nucleus:heated to maximal T
cooling by evaporationof neutrons emission of gamma's
fission
•nonlinear motion in shapes and T•T varies in time • treat in parallel:
• d(Q,P,t)• T=T(t)• Evaporation
• for d(Q,P,t) :•FokkerPlanck•Langevin
• Quasistatic picture
Discovery of Elements 118 and 116..... scientists from the Berkeley Lab and Oregon State University report the observation of superheavy elements in the reaction 86Kr + 208Pb performed at LBNL's 88Inch Cyclotron.... http://user88.lbl.gov/element118.html
Theoretical calculation of binding energies P. Möller, J.R. Nix and A. Sierk, Los Alamos Nat. Lab.
Discovery of Elements 118 and 116..... scientists from the Berkeley Lab and Oregon State University report the observation of superheavy elements in the reaction 86Kr + 208Pb performed at LBNL's 88Inch Cyclotron.... http://user88.lbl.gov/element118.html
Theoretical calculation of binding energies P. Möller, J.R. Nix and A. Sierk, Los Alamos Nat. Lab.
art of the game:
• choose right projectiletarget combination to reach „island of stability“• initial energy:
•big enough to overcome fusion barrier•small enough to keep excitation of fused system small
one is happy if s.h. lives for msec Berkeley, GSI, Dubna
Table of contents• Elements of nuclear physics
– shell model vs compound nucleus
• Concept of nuclear temperature: false or true?– consequences of thermal isolation
• Transport theory – application in fission and heavy ion collisions– origin of irreversibility, nature of dissipation– fluctuating forces
• Summary, open problems
linearize mean field locally i.e. H(x,p;Q(t)) in QQ_0treat coupling within linear response theory
assume:nucleonic d.o.f. to be close to thermal equilibrium(quasistatic picture)
transport coefficients determined by local response function
H.H. , P. Siemens, A.S. Jensen (`7484)H.H. and many others afterwards
deformed mean fields (parameterized by shape variables Q(t))
WoodsSaxonpotential
Cassini ovaloidsV. Paskevich
given density distribution folded with Yukawa interactionbecause of short range:
Berkeley Los Alamos group
potential deformation
follows that of densityselfconsistency condition
microscopic origin of dissipation
dissipation for Qmode if and only if coupling induces
real transitions betweeen states of nucleonic motion
slow motion transitions of small energy
microscopic origin of dissipation
dissipation for Qmode if and only if coupling induces
real transitions betweeen states of nucleonic motion
slow motion transitions of small energy
<< level spacing: NO dissipation
depends on size of system: i.p.m.: dissipation can exist only in macroscopic limit
„philosophy“ of wall formula (Swiatecki et al.)nucleonic motion treated in (semi)classical or macroscopic limit
same problem as for Ohmic resistance for DC in metal clusters (A. Kawabata and R. Kubo (1966), D.M. Wood and N.W. Ashcroft (1982) ....)
microscopic origin of dissipation
at finite T nucleonic states have finite width:
dashed curve: i.p.m
true relaxation onlywith finite s.p. widths(fully drawn lines)
nucleonic relaxation time about 0.5 to 0.2 h/MeV << collective by factor 5
timedependent response functions
Tdependence of nuclear transport
schematic behavior: details depend on parameters for widths etc
mind: transport properties (for average motion)
involve friction, inertia and local stiffnes
Tdependence of nuclear transport
schematic behavior: details depend on parameters for widths etc
experimental evidence: damping increases strongly with T
(P.Paul, M. Thoennessen ..)
in agreement (solely)
with our theoretical model
(different to wall or two body friction)
mind: transport properties (for average motion)
involve friction, inertia and local stiffnes
M. Thoennessen and G.F. Bertsch PRL 26 (1993) 4303I. Diószegi et al. PRC 61 (2000) 024613
H. H., F.A. Ivanyuk, C. Rummel and S.Yamaji, PRC 64 (2001) 054316
quantum transport
• CaldeiraLeggett model studied with functional integrals schematic potential, constant inertia bilinear in nuclear and collective d.o.f. oscillator bath with constant shapes selfconsistency requires:many beautiful results by many groups (H. Grabert, P. Hänggi, U. Weiss...)
not applicable in nuclear physics
• Quantum optics: transport equations (for real time propagation) of FokkerPlanck type with quantal diffusion coefficients
• in early 80‘ties: similar type of equation derived by us for nuclear case within LHA: problem: restricted to weak damping
not really applicable in nuclear physics
quantum transport
• transport equation (for real time propagation) of FokkerPlanck type with quantal diffusion coefficients (for arbitrary damping)– developed for LHA in late 80‘ties early 90‘ties– and applied to obtain quantum corrections to Kramers‘ rate (H. H., G.L. Ingold and M. Thoma, PLB 317 (1993) 489)
requires analytic continuation of quantal fluctuation dissipation theorem to unstable modes (in barrier region ) breaks down below critical temperature T_0 same feature in CaldeiraLeggett model for real time propagation
quantum transport
• Doctoral thesis C. Rummel (TUM ´04) employ theoretical means developed for CaldeiraLeggett model
except starting point to account for selfconsistency– residual interaction of separable nature– introduce collective coordinate and effective action by Hubbard Stratonovich trick – evaluate imaginary part of partition function (instability) to get decay rate– generalize harmonic approximation
• expansion to fourth order• modification of FeynmanKleinert varitional procedure
• problem: requires constant temperature
Table of contents• Elements of nuclear physics
– shell model vs compound nucleus
• Concept of nuclear temperature: false or true?– consequences of thermal isolation
• Transport theory – application in fission and heavy ion collisions– origin of irreversibility, nature of dissipation– fluctuating forces
• Summary, open problems
Summary• at finite T nuclear collective motion: a problem of transport theory unique features: nuclei are small, thermally isolated and self
bound• temperature varies in time because of:
– transfer of energy from collective motion to „heat bath“– evaporation of light particles and photons
• for average motion: – pairing implies weak damping or no damping at all– with increasing T damping increases and may become very strong– as (eventually ?) seen in experiments (analyzes require reconsideration)– transport properties involve effects from inertia and thermostatic energies– related to selfconsistency problem – it is vital to treat nucleonic motion qunatum mechanically and not in macroscopic limit
Summary• dynamics of fluctuations on classical level
– structure of equation like known from Kramers– input determined by transport coefficients of average motion
• extension to quantum physics within LHA– allows for treating varying temperature– modified diffusion coefficients– possible above T_0 >0 of order of less than 0.5 MeV
• beyond LHA with functional integrals – requires constant T – problems with decent introduction of inertia